Hellas Planitia

Hellas Planitia is the largest impact basin on Mars;
it lies in the planet's southern highlands. Hellas Planitia – the
roughly circular feature that dominates the image to the right – is
about 2,300 kilometers (1,430 miles) across, or half the size of the United States.
It was formed by a giant impact during the late
heavy bombardment period of the solar system, some 3.9 billion years
ago. Hellas Planitia also contains the lowest elevation point on Mars, reaching
about 9 kilometers below the surrounding highlands.

The Soviet probe Mars 2 crashed into
Hellas Planitia at approximately 45°S 302°W in 1971, thus becoming the
first human-made artefact to reach the martian surface.